struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2)
{
    struct ListNode* head = NULL, * tail = NULL;
    if (list1 == NULL)
        return list2;
    if (list2 == NULL)
        return list1;
    while (list1 && list2)
    {

        if (list1->val < list2->val)
        {
            if (tail == NULL)
                head = tail = list1;
            tail->next = list1;
            tail = list1->next;
            list1->next;
        }
        else
        {
            if (tail == NULL)
                head = tail = list2;
            tail->next = list2;
            tail = list2->next;
            list2->next;
        }
    }
    if (list1)
        tail->next = list1;
    if (list2)
        tail->next = list2;
    return head;
}